1. Field of the Invention
The present invention relates to a method of purification of wet process phosphoric acid. More particularly, it relates to a process for producing purified phosphoric acid from wet process phosphoric acid with the industrial advantage that organic impurities present in the wet process phosphoric acid are removed by a solvent extraction method.
2. Description of the Prior Art
The wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid and separating calcium sulfate usually contains impurities such as iron, aluminum, calcium, sodium, magnesium, titanium components and sulfuric acid, fluorine and silica components which are derived from the raw materials of the phosphate rock and the sulfuric acid. Accordingly, the wet process phosphoric acid has been mainly used for the preparation of fertilizers. However, substantial research and development work has been directed to the production of purified phosphoric acid useful for industrial purposes, pharmaceutical purposes and food additive applications, by extracting the wet process phosphoric acid with an organic solvent which dissolves phosphoric acid and which is not very miscible with water such as alcohols, e.g. n-butanol, isobutanol and isoamyl alcohol; ketones, e.g. methylethyl ketone and methyl isobutyl ketone; ethers, e.g., isopropyl ether; phosphoric acid esters, e.g., tributyl phosphate and amines, and back-extracting phosphoric acid from the solvent layer with water.
Normally, uncalcined phosphate rock is used for the economic production of wet process phosphoric acid. Therefore, relatively large amounts of organic impurities are incorporated in the wet process phosphoric acid which becomes a brown to black brown color because of the presence of the organic impurities. When the wet process phosphoric acid is used for the fertilizer, the organic impurities do not cause any problems. However, when a purified phosphoric acid used for industrial purposes, pharmaceutical purposes and as a food additive is produced by purifying the wet process phosphoric acid by a solvent extraction process, if a large amount of the organic impurities are present in the wet process phosphoric acid, the following disadvantages arise which inhibit smooth operation:
(a) The phosphoric acid extraction efficiency and the impurity separation efficiency are lowered.
(b) The speed of phase separation of the two liquid phases in the extraction process is very slow. Moreover, separation of the two liquid phases is not easily attained by using the conventional mixer-settler type extractor etc. and it is therefore necessary to use expensive equipment such as a centrifugal extractor to facilitate phase separation and extraction.
(c) Sometimes a stable emulsion is formed whereby the separation of the two liquid phases is impossible.
(d) A solid mass of organic impurity forms at the interface between the two liquid phases. This leads to the formation of a scale which can inhibit operation of the system for long periods of time.
For the reasons above, it has been difficult to use wet process phosphoric acid which contains organic impurities in solvent extraction processes.
Various processes for purifying wet process phosphoric acid by selectively removing the organic impurities from the wet process phosphoric acid directly or the acid extraction section have been proposed. For example, it has been proposed to pass wet process phosphoric acid which contains organic impurities through a column filled with granular active carbon before it is fed to an acid extraction process. However, in this process, the column filled with granular active carbon is frequently clogged with fine insoluble materials present in the wet process phosphoric acid as well as precipitates of fine gypsum and silicofluorides. These fine insoluble materials and the precipitates cover the surface of the active carbon which diminish the activity of the carbon as well as adversely effect the recovery coefficient. Thus, the process can not be employed for industrial purposes. When powdered active carbon is used as the carbon source instead of granular active carbon, a large amount of expensive active carbon must be used and a step in the process is required to separate the active carbon. Moreover, the treatment of the waste active carbon is a problem from the viewpoint of the environmental pollution. Accordingly, the process is not economical for industrial purposes.
Another proposed process for removing organic impurities in the solvent extraction process involves distilling the solvent phase which contains substantially no phosphoric acid such as recycled solvent, or washing the recycled solvent with an aqueous solution of an alkali metal hydroxide, or contacting the recycled solvent with active carbon or an anion exchange resin. However, in the acid extraction method, normally from 4 to 20 times by volume of the organic solvent is used to wet process phosphoric acid thereby necessitating the use of large size equipment as well as large amounts of a treating agent. Moreover, large amounts of organic solvent must be used and the loss of expensive organic solvent is substantial. Accordingly, these processes are not economically advantageous so that a need continues to exist for an economic and effective process for removing the organic impurities from wet process phosphoric acid.
A study has been conducted on the behavior of organic impurities in the solvent extraction process in order to develop a process for purifying wet process phosphoric acid by the solvent extraction process under conditions in which organic impurities are selectively removed. As a result, it has been found that most of the organic impurities present in wet process phosphoric acid are easily extracted into the organic solvent and that the organic impurities are the cause of the above-described problems. Moreover, it has been discovered that the organic impurities are substantially not back-extracted with water and therefore accumulate in the solvent phase in the acid extracting section during long periods of operation. Still further, it has been found that the separation of the two liquid phases does not deteriorate under some specific conditions even though large amounts of the organic impurities can be present.